1. Field of the Invention
This invention relates to crystallized glasses suitable for substrates which are used for information recording media, such as magnetic disks, optical disks and optical magnetic disks, substrates for information recording media composed of this crystallized glass, and information recording media using said substrate for information recording medium.
2. Description of the Related Art
Major components of magnetic storage devices of electronic computers and the like are a magnetic recording medium and a magnetic head for reconstruction of magnetically recorded information. Flexible disks and hard disks have been known as magnetic recording media. As substrates for hard disks, aluminum alloy has been mainly used. Recently, flying height of magnetic heads is markedly reduced as hard disk drivers for notebook personal computers are made smaller and their magnetic recording density made higher. Accordingly, extremely high precision has been demanded for the surface smoothness of magnetic disk substrates. However, it is difficult to produce smooth surface more than a certain level of precision with an aluminum alloy. That is, even though it is polished using highly precise abrasives and processing apparatuses, the polished surface may suffer from plastic deformation because of the low hardness of the alloy. Even if the aluminum alloy is plated with nickel-phosphorous, the surface roughness Ra cannot be made equal to or less than 5 Å (angstrom). In addition, as hard disk drivers are made smaller and thinner, a further smaller thickness of substrates for magnetic disks is also strongly desired. However, it is difficult to produce such a thin disk with an aluminum alloy having a certain strength defined by specification of hard disk drivers because of low strength and stiffness of aluminum alloy.
Therefore, glass substrates for magnetic disks of which high strength, high stiffness, high impact resistance and high surface smoothness are required have been developed. Among these, chemically reinforced glass substrates whose surfaces are strengthened by the ion exchange technique, crystallized glass substrates subjected to crystallization treatment and the like have been known well.
As a chemically reinforced glass substrate by ion-exchange, for example, a glass disclosed in Japanese Patent Unexamined Publication No.Hei. 1-239036 (JP-A-239036/89) has been known. This chemically reinforced glass substrate is such a glass substrate for magnetic disks as the glass containing, indicated in terms of % by weight, 50-65% of SiO2, 0.5-14% of Al2O3, 10-32% of R2O where R is an alkali metal ion, 1-15% of ZnO and 1.1-14% of B2O3 is reinforced by forming a crushing stress layer on the glass substrate with an ion exchange method by an alkali ion.
In addition, as a crystallized glass, for example, a glass disclosed in Japanese Patent Examined Publication No.2516553 is exemplified. This crystallized glass is such a crystallized glass for magnetic disks which contains, indicated in terms of % by weight, 65-83% of SiO2, 8-13% by Li2O, 0-7% of K2O, 0.5-5.5% of MgO, 0-5% of ZnO, 0-5% of PbO (provided that MgO+ZnO+PbO is 0.5-5%), 1-4% of P2O5, 0-7% of Al2O3 and 0-2% of As2O3+Sb2O3, and contains micro crystalline particles of Li2O.2SiO2 as main crystals.
Moreover, a crystallized glass is also disclosed in Japanese Patent Unexamined Publication No.Hei.7-291660 (JP-A-291660/95). This crystallized glass is obtained by heat treatment after fusion and forming a glass which consists of, indicated in terms of % by weight, 38-50% of SiO2, 18-30% of Al2O3, 10-20% of MgO, provided that having a composition containing, indicated in terms of weight ratio, 1.2-2.3 of Al2O3/MgO, 0%-5% of B2O3, 0%-5% of CaO, 0%-5% of BaO, 0%-5% of SrO, 0.5%-7.5% of ZnO, 4%-15% of TiO2, 0%-5% of ZrO2 and 0%-2% of As2O3 and/or Sb2O3. This glass is a cordierite based crystallized glass characterized by containing cordierite based crystals as crystals. Moreover, a substrate for magnetic disks composed of this crystallized glass is also disclosed.
In addition, a crystallized glass is also disclosed in Japanese Patent Unexamined Publication No.Hei.9-77531 (JP-A-77531/97) (U.S. Pat. No. 5,476,821). This crystallized glass is a ceramic product having the Young's modulus in the range of from about 14×106-about 24×106 psi (96-165 GPa) and the fracture toughness of more than 10 MPa.m1/2. In addition, this crystallized glass consists of crystalline phase laminated body which mainly consists of crystals having a spinel structure and a uniform size and dispersing uniformly in a siliceous-rich residual matrix. This is a glass ceramic which substantially consists of, indicated in terms of % by weight using oxides as a standard, 35-60% of SiO2, 20-35% of Al2O3, 0-25% of MgO, 0-25% of ZnO, 0-20% of TiO2, 0-10% of ZrO2, 0-2% of Li2O and 0-8% of NiO. This has at least about 10% of a total amount of MgO+ZnO and may contain equal to or less than 5% of an optional component selected from the group composed of BaO, CaO, Pbo , SrO, P2O5, B2O3 and Ga2O3, in the range of from 0 to 5% of R2O selected from the group consisting of Na2O, K2O, Rb2O and Cs2O, and in the range of from 0 to 8% of transition metals. In the case of containing less than about 25% of Al2O3, this is a glass ceramic having a composition in which the total amount of TiO2+ZrO2+NiO is equal to or more than 5%. In above publication, the substrate for magnetic disks consisting of this glass ceramic is disclosed.
In addition, a crystallized glass is also disclosed in U.S. Pat. No. 5,491,116. This crystallized glass is a glass ceramic product having the fracture coefficient of at least about 15,000 psi, the Knoop hardness exceeding about 760 KHN, the Young's modulus of more than about 20×106 psi and the fracture toughness of more than 1.0 MPa.m1/2. The main crystals of the crystallized glass are enstatite or its solid solution and spinel (spinel structure crystal), and the crystallized glass contains at least 92% of the composition substantially composed of, indicated in terms of % by weight, 35-60% of SiO2, 10-30% of Al2O3, 12-30% of MgO, 0-10% of ZnO, 5-20% of TiO2 and 0-8% of NiO. Moreover, the substrate for magnetic disks composed of this crystallized glass is also disclosed. It is to be noted that the same glass as the crystallized glass disclosed in aforementioned patent is also disclosed in Journal of Non-Crystalline Solids 219(1997) 219-227.
However, along with making hard disks smaller and thinner and making recording density higher, it is rapidly developed to make flight height of magnetic heads smaller and revolution speed of disks higher. Thereby, substrate materials are more strictly required the strength, the Young's modulus, the smoothness of the surface and the like. In particular, by making the information recording density of 3.5-inch hard disks for personal computers and severs higher, the surface smoothness and the surface flatness of the substrate materials are strictly required. In addition, corresponding to the higher data processing speed, it is required to set the winding number of the disks equal to or higher than 10,000 rpm. Thus, the requirement for stiffness of substrate materials becomes increasingly severer, and the limitation of conventional aluminum substrates already becomes obvious. In future, as long as it is necessarily demanded to make the capacity of hard disks higher and to make the revolution speed of hard disks higher, it is clear that the substrate materials for magnetic recording medium is strongly required to exhibit higher Young's modulus, higher strength, more excellent surface flatness, higher impact resistance and the like.
However, such a chemically reinforced glass as disclosed in Japanese Patent Unexamined Publication No.Hei.1-239036 (JP-A-239036/89) mentioned above has the Young's modulus of about 80 GPa, therefore, it cannot meet the strict demand for hard disks in future. As for conventional chemically reinforced substrate glasses, alkali ions are introduced in a large amount in the glass for ion exchange, so that the reinforced glasses mostly have the low Young's modulus (90 GPa). Moreover, due to also having low stiffness, it cannot meet 3.5-inch high-end disk substrates and thinner disk substrates. In addition, the glass chemically reinforced by ion exchange contains large amount of alkali components. Thus, if it is used for long hours under the circumstance of high temperature and high humidity, alkali ions deposit from parts including thin magnetic films or exposing glasses, such as a pinhole part of a magnetic film or a circumference of a magnetic film. It has a disadvantage that this triggers a corrosion or decomposition of the magnetic films. In the producing process of the magnetic recording medium, after providing a magnetic layer on the glass substrate, certain heat treatment may be carried out in order to improve characteristics such as coercive force of the magnetic layer. However, the conventional ion-exchanged reinforced glass mentioned above has at most 500° C. of the glass transition temperature, so that it has poor heat resistance. Thereby, it has a problem that higher coercive force cannot be obtained.
In addition, the conventional crystallized glass as disclosed in Japanese Patent Publication No. 2516553 mentioned above is superior a little in the Young's modulus and heat resistance than aforementioned chemically reinforced glass substrate. However, the surface roughness is equal to or higher than 10 Å, thereby the surface smoothness is poor, so that it is limited to make the flying height smaller. Therefore, it has a problem that it cannot meet higher magnetic recording density. Moreover, the Young's modulus is about from 90 to 100 GPa at most, so that it also cannot meet 3.5-inch high-end disk substrates and thinner disk substrates.
In addition, the crystallized glass disclosed in Japanese Patent Unexamined Publication No.Hei.7-291660 (JP-A-291660/95) mentioned above has the Young's modulus of about from 100 to 130 GPa, therefore, it cannot be said that it is sufficient. Moreover, it has only such a surface smoothness as having the Young's modulus of about 8 Å, resulting in poor smoothness. Additionally, the temperature of glass liquid phase is high which is 1400° C., so that it has a disadvantage of difficulty in producing.
Moreover, the crystallized glass disclosed in Japanese Patent Unexamined Publication No.Hei.9-77531 (JP-A-77531/97) mentioned above has a disadvantage of large difficulty in polishing because the main crystals are spinel.
Moreover, a large amount of enstatite is contained together with spinel in the crystallized glass disclosed in U.S. Pat. No. 5,491,116 and Journal of Non-Crystalline Solids 219(1997) 219-227. Accordingly, it can be considered that easiness in polishing is more improved than the crystallized glass disclosed in Japanese Patent Unexamined Publication No.Hei.9-77531 (JP-A-77531/97). However, because spinel is still contained therein, it is difficult to say that it has sufficient polishing characteristics. That is, it still takes a long time for polishing required for obtaining desirable surface roughness, so that it has a problem of inferior productivity.
Moreover, because a glass disclosed in Japanese Patent Publication No.2648673 is a fire-resistant glass ceramic for the purpose of using temperature equal to or higher than 1200° C., it is difficult to be used as a substrate for information recording medium. That is, it is difficult to be produced due to high melting temperature, moreover, the surface smoothness required for information recording medium cannot be obtained due to large crystal size.